1. Field of the Invention
The present invention relates to a gas detecting device installed in a vehicle for detecting a gas concentration in the air.
2. Description of the Prior Art
A gas detecting device for a vehicle has been proposed such as in Japanese First (unexamined) Utility Model Publication No. 4-72009 and Japanese First (unexamined) Patent Publication No. 4-123919, wherein a gas sensor is received in a casing which forms therein an air passage.
The gas detecting device of this type is provided with flow-velocity control sections at an inlet and an outlet of the casing for controlling an Air-flow velocity at substantially a constant small value. With this arrangement, even when a wind velocity outside the casing changes, a wind velocity within the casing (air passage) can be held substantially constant at a small value. Thus, the gas sensor arranged in the casing can detect a contamination degree of the air reliably without being affected by the wind velocity outside the casing.
On the other hand, in the foregoing gas detecting device, since the outside air flowing into the casing directly hits the gas sensor, it is highly possible that dust in the air adheres to a detecting surface of the gas sensor and that the detecting surface is subjected to invasion of water or moisture.
In view of this, it has been proposed to arrange a filter upstream of the gas sensor. The filter has an air-inlet side end surface, an air-outlet side end surface and side surfaces extending along the air passage and connecting the air-inlet side end surface and the air-outlet side end surface to each other. The air is introduced into the air passage via the inlet of the casing and flows into the filter via the air-inlet side end surface and out of the filter via the air-outlet side end surface. When the moisture or water enters the air passage via the inlet of the casing to wet the filter, a water film is likely to be formed at the air-inlet side end surface of the filter so that the air-inlet side end surface of the filter can not be used as an air-passing surface. As a result, the necessary air is not supplied to the gas sensor so that accurate detection of a gas concentration is rendered difficult.